The Strange Spires of Callisto

August 22, 2001: Three months ago when
NASA's adventurous Galileo spacecraft skimmed a mere 138 km above
the surface of Jupiter's moon Callisto, onboard cameras captured
the sharpest pictures ever of that moon's mysterious landscape.
Scientists have since examined the images, and what they found
is surprising. Callisto is peppered with strange icy features
-- spires that seem to be slowly eroding on a world long considered
changeless and dead.

"We haven't seen terrain like this before. It looks like
erosion is going on, which is pretty surprising," said James
Klemaszewski (Academic Research Lab) who, along with David Williams
and Ronald Greeley (Arizona State University), is analyzing Galileo's
latest pictures of Callisto.

Above: Bright scars on a darker surface testify to a
long history of impacts on Jupiter's moon Callisto in this image
of Callisto from NASA's Galileo spacecraft. The picture, taken
in May 2001, is the only complete global color image of Callisto
obtained by Galileo. [more
information]

Callisto's icy surface is the most heavily cratered place
in the Solar System. There are no volcanoes or winds and rain
to obliterate landforms. So, craters that form as the result
of occasional impacts with meteorites are very long-lasting.
Planetary scientists call terrains like Callisto's "old."
Earth's much-weathered surface, on the other hand, is very "young."
For billions of years, little has changed on Callisto other
than the relentless accumulation of craters -- or so researchers
thought.

Galileo's latest images are painting a different picture.
Callisto's knobby terrain is unlike any seen before on Jupiter's
moons --and it's not entirely unchanging.

During the Callisto flyby, Galileo's camera saw spire-like
"knobs" jutting 80 to 100 meters (260 to 330 feet)
high, consisting perhaps of material thrown outward from a major
impact billions of years ago. The knobs are very icy, but they
also harbor some darker dust. The dark material seems to be sliding
down the knobs and collecting in low-lying areas.

"They are continuing to erode and will eventually disappear,"
Klemaszewski said. One theory for an erosion process is that,
as some of the ice turns into vapor, it leaves behind dust that
was bound in the ice. The accumulating dark material could absorb
enough heat from the distant Sun to warm the surrounding ice
and keep the process going.

Right:
The top inset shows some of Callisto's many icy knobs. The bottom
image shows a dark-plained region where erosion has essentially
ceased, allowing impact craters to persist and accumulate. [more
information]

The eroding spires of Callisto are just one of the moon's
riddles. Indeed, with a diameter of 4,800 km -- nearly the size
of Mercury -- Callisto is a bona fide world of its own with mysteries
befitting a full-fledged planet. For example, magnetic readings
picked up by Galileo when it encountered Callisto in 1996 and
1997 suggest that the pock-marked satellite harbors one of the
solar system's biggest salty oceans. But the water, if it's really
there, does not lie atop the frigid surface. Instead, Callistan
oceans would be hidden deep below the moon's spikey, slowly
eroding crust -- another tantalizing puzzle for future spacecraft
and explorers.